In recent years, plastics have been widespread in the fields of various domestic appliances, office automation equipments, etc. with the improvement of accuracy and properties due to the development of molding techniques, new plastic materials, and the like.
Upon a particular consideration of gears that are sliding parts of these equipments, plastics are widely employed chiefly as gears of small transmission torque or small-sized gears from the standpoint of decreased cost resulted from mass production, corrosion resistance, reduced noise, and the like in spite of their inferiority in stiffness or strength.
Polyacetal resins have been generally used for the particular use as precise gear of small size because of their relatively well-balanced characteristics as required for gear materials, such as molding accuracy, fatigue strength, etc.
Polyacetal itself is an engineering resin excellent in sliding properties, and studies have been directed to broadening of utility of polyacetal by incorporating carbon black thereinto to thereby impart thermal conductivity.
However, a polyacetal composition having incorporated therein carbon black in an amount sufficient to obtain required properties has suffered many problems, such as decomposition of polyacetal during kneading, serious reduction in moldability, extreme deterioration in molding appearance, and the like.
In addition, the sliding part (e.g., gears) comprising a polyacetal resin makes a disturbing noise when used under severe conditions, i.e., a high load and a high speed.
With the strong demand for reduction of noise caused by various domestic appliances, office automation equipments, etc., it has been keenly demanded to improve molding accuracy, abrasion resistance, and sliding properties of sliding parts, e.g., gears, bearings, cams, etc.
In order to meet these requirements, it has been proposed to add an amide compound and polyethylene wax to polyacetal containing conductive carbon black as disclosed in U.S. Pat. No. 4,555,357. This proposal achieves some improvements on the above-described problems but still has another disadvantage that the sliding properties inherently possessed by polyacetal deteriorate.